First, when you're running the movie in Flash (the editor) itself, then animations won't work. So "enter" will show no animation. You have to export the movie (CTRL+enter) or menu:control>test movie. If that's not the cause of the problem, how many frames has your movie? How many frames has the layer where you put the animation on? Try adding enough frames. Use a new layer without any keyframes, and use that for all animations that should loop. If those animations should run asynchronous, then add a layer for each animation, and let them start from a keyframe.
Thanks heaps for that!
I didn't even think of the test movie... I was hitting enter, looking through the timeline etc and seeing it wasn't running. I just expected something like this to work (like other software such as AfterEffects usually would).
Back on track now!
I know all about it! I had the same problem about a week ago...